Level controls are used in many fluid systems to maintain the volume of liquid in a tank or other containment vessel substantially constant. In instances where the tank or the like receives an inflow of liquid, the level control operates by establishing a matching discharge flow. In certain systems of this kind it is desirable that the discharge flow be drawn from below the surface of the liquid. Thus the desired liquid level cannot be maintained by simply providing an overflow passage or drain opening in the wall of the vessel at the desired liquid level.
Chemostats for culturing microorganisms in a liquid medium are one example of such systems. Maintenance of the microbial culture usually requires a continuous or at least periodic inflow of liquid medium to the chemostat vessel. A matching controlled discharge flow of liquid from the chemostat tank is required to avoid overflow and to maintain the culture volume constant. It is not usually practical to accomplish this by simply pre-establishing matched fixed inflow and outflow rates. Even a slight imbalance in the initial adjustment or an imbalance which arises in the course of operation from any of various possible causes will, over an extended period of time, result in a significant rise or fall of liquid level in the chemostat. Thus as a practical matter the liquid discharge components must include some arrangement for varying the outflow when necessary to maintain a precise balance with inflow.
A surface layer of scum, foam or other floating matter may form on the chemostat liquid. Thus it is desirable that the discharge flow be drawn from a level below the surface of the liquid. This reduces clogging in the discharge flow path and also provides an effluent having a composition representative of the bulk of the liquid medium within the chemostat and which is more useful for analysis than a sample taken from the atypical surface layer.
Prior chemostat liquid level controls that provide for subsurface draw off often rely on an inverted siphon or, alternately, have a discharge pump or solenoid operated pinch valve controlled by a level sensor probe that is typically electrical in part. Each of these types of prior level control is subject to significant problems and disadvantages.
An inverted siphon in this context requires that an opening be made in the chemostat vessel wall at or below the desired liquid level. Chemostat tanks are typically formed of glass and are thus difficult to perforate. Further, the junction between the wall and siphon tube is prone to breakage and in some cases to leakage and the protruding tube may interfere with operations to be performed or other apparatus in the vicinity. Because the discharge flow of a siphon is generated by gravity, it tends to be less forceful than is desirable and such siphons are prone to clogging.
Consequently, the systems which use a pump or solenoid controlled valve and electrical liquid level probe have usually been considered to be the preferred type. Performance requirements for these instruments are frequently severe. In many cases, the probe must withstand repeated autoclaving. Operation under either aerobic or anaerobic conditions may be required and the probe must accurately sense liquid level even in the presence of foam, scum or other floating material. Electrical probes for such usage are very costly and may have adverse effects. In one instance, for example, use of an electronic liquid level probe was found to induce extensive electrolytic corrosion of submerged stainless steel elements in a chemostat tank which in turn released toxic hexavalent chromium into the bacterial culture.
Thus a more reliable and trouble free form of liquid level control would be highly advantageous in chemostats and in other similar fluid systems where similar problems and requirements are experienced. Level control should preferably be accomplished by simple and economical components that do not require the use of an electrical sensor probe or the like but which do coact with a discharge pump to provide for a forced outflow drawn from below the surface of the liquid. Preferably, such a system should not necessarily require perforation of the wall of the liquid containment tank or the like.
The present invention is directed to overcoming one or more of the problems as set forth above.